The Oso Wastewater Treatment Plant is the largest of the six plants owned by the City. This plant serves the southside of the city where over 50 percent of the population lives. It is located closed the Texas A&M University - Corpus Christi. It was completed in 1941 and has been expanded several times. The last major expansion was completed in 1982 when the plant's capacity was increased to treat 16.2 mgd.

The plant is permitted for a 20 milligram/liter (mg/l) BOD5 and 20 mg/l TSS. The plant is presently treating about 11.3 mgd, on average. During the past seven years, the plant has won three Gold and four Silver Awards. It also won the U.S. Environmental Protection Agency's Excellence Award for the year 1987.

The plant is manned twenty four hours per day, seven days a week. The Oso Plant is an activated sludge plant and is operated in a contact stabilization mode. It has two process trains, each capable of treating about 11.3 mgd. The sludge (Bio-Solids) is aerobically digested and processed through the belt filter presses before it is disposed at the City owned landfill. The dewatered sludge consists of approximately 11 to 15 percent solids. About 10 dry tons of sludge is processed per day. This facility operates under an authorized permit issued by Texas Commission on Environmental Quality.